Isoëtes azorica Durieu ex Milde - ein Endemit der Azoren - Vegetations- und Standortsverhältnisse, Gefährdung und Schutzmaßnahmen
For the first time the vegetation and site conditions for the occurrence of Isoëtes azorica Durieu are described for seven different crater lakes of the Western and Central group of the Azores Islands. Isoëtes azorica, a rare endemic species of the Azores, colonizes only a few lakes to a depth of 2.5 m in low-species dominance stands which are designated Isoëtetum azoricae and classified to belong to the class Littorelletea. - Nowadays a great number of lakes on the larger islands for which the occurrence of Isoëtes stands in former times could be established are only colonized by extensive poor-species dominance stands of Littorella uniflora. Various succession stages in the change of the vegetation from Isoëtes lakes to the floating vegetation of Potamogeton polygonifolius and Eleogiton fluitans stages with an abundance of Juncus bulbosus are reported. These are weakly acid, low-lime, oligotrophic lakes of clear water colour, where the latter is due to the low organic load. The total hardness is in the "extremely soft" range. The ion content of the waters is determined by sodium and chloride. The enrichment of nutrients and grazing of the banks and shallow water areas entail an extinction and eradication of Isoëtes occurrences. - Criteria of danger and required measures of protection are discussed.